Scramble with "scrambled" screen

ManiN

Member
Joined
Jul 31, 2009
Messages
727
Reaction score
7
Location
Washougal, Washington
Got a board that shows just garbage and does nothing else. Looked around for anything obvious but nothing. Swapped the Z80 with no change.

40957c1d8c426c7d1a1cacfea005382f.jpg


Any ideas or experiences with this? Maybe tip on where to start?
 
Look to see if the crystal is knocked off of if it there and you touch it and it makes noise or feels lose replace it.
 
Would I be looking for maybe a dead pin on 22 for that? I have a logic probe with only basic experience of it. As a recap, I did swap the z80 with no change, but could verify the trace is actually showing the right results in the event of a break or bad socket I guess.
 

Attachments

  • pinout.png
    pinout.png
    55.9 KB · Views: 24
looked at this some more. Tried using my logic probe which I think is crapping out cause it no longer makes noise... :(

Poked at a few TTL chips and there were a few that had what I thought were dead pins close to the CPU, but they also didn't have traces to those pins, so didn't think about that.

I definitely think there is a watchdog reset issue going on. This can speak to a crystal issue in which I poked at both on this board set, but no change just poking them. I don't have spares so I can't swap them out without a shotgun guess.

Tried looking for bad traces and or bad solder on any of the chips. Nothing.
 
Check pin 26 on the CPU see if it is stuck low. Also verify pin 6 is not stuck it should be changing at a fixed high frequency
 
Looks like it's not coming out of reset unless it goes down first for a couple of microseconds when you first apply power. Does the screen change at all or is it static.

At least your video is stable (judging from the picture) and showing stuff. The garbage on the screen is just whatever is in VRAM.

Yvo
 
Back
Top Bottom